cmark 0.17

• Stripped out all JavaScript related code and documentation, moving it to a separate repository (https://github.com/jgm/commonmark.js).
• Improved Makefile targets, so that cmake is run again only when necessary (Nick Wellnhofer).
• Added INSTALL_PREFIX to the Makefile, allowing installation to a location other than /usr/local without invoking cmake manually (Nick Wellnhofer).
• make test now guarantees that the project will be rebuilt before tests are run (Nick Wellnhofer).
• Prohibited overriding of some Makefile variables (Nick Wellnhofer).
• Provide version number and string, both as macros (CMARK_VERSION, CMARK_VERSION_STRING) and as symbols (cmark_version, cmark_version_string) (Nick Wellnhofer). All of these come from cmark_version.h, which is constructed from a template cmark_version.h.in and data in CMakeLists.txt.
• Avoid calling free on null pointer.
• Added an accessor for an iterator's root node (cmark_iter_get_root).
• Added user data field for nodes (Nick Wellnhofer). This is intended mainly for use in bindings for dynamic languages, where it could store a pointer to a target language object (#287). But it can be used for anything.
• Man renderer: properly escape multiline strings.
• Added assertion to raise error if finalize is called on a closed block.
• Implemented the new spec rule for emphasis and strong emphasis with _.
• Moved the check for fence-close with the other checks for end-of-block.
• Fixed a bug with loose list detection with items containings fenced code blocks (#285).
• Removed recursive algorithm in ends_with_blank_line (#286).
• Minor code reformatting: renamed parameters.